Grape growers as well as my visual observations confirm that some vine- yards are experiencing powdery mildew outbreaks. If your vineyard is experiencing a powdery mildew outbreak after the application of a DMI fungicide (Frac Code 3) then consider that the powdery mildew may have developed resistance. Powdery mildew resistance to DMI inhibitors was confirmed in Missouri in 2021. Some of the common DMI fungicides include: Mettle, Procure, Rally, TebuStar (tebuconazole) and Vintage. Alternative fungicides to control powdery mildew are: Merivon Xemium (Frac 7+11), Quintec (Frac 13), Topsin M (Frac 1), Torino (Frac U6) and Vivando (U8).
Some of these alternative fungicides have lengthy pre–harvest intervals (Table 1). Be aware of these PHI’s as harvest nears. Other alternatives for control are to apply a strobilurin fungicides such as Abound, Flint, Pristine or Sovran. Do not apply stobilurin fungicides if you have already made two applications this growing season. Also do not apply strobilurin fungicides if your grapevines are heavily inundated with powdery mildew. Extremely high powdery mildew populations coupled with a very selective fungicide such as the strobilurins increases the potential for selecting for powdery mildew resistant populations. In addition, sulfur is an excellent preventative for powdery mildew. However, sulfur has limited usefulness if powdery mildew has become established throughout a vineyard. Be sure to keep sulfur off of sulfur sensitive cultivars such as Chambourcin and Norton. — By Dean Volenberg, University of Missouri Grape & Wine Institute